The present invention relates to the field of valve positioners and more particularly to a gain adjustment mechanism for a valve positioner.
Valve positioners employing a feedback system operating on so-called "force-balance" principles have been in existence for years. Such valve positioners may be used to control the flow rate through a system valve according to a control signal sent to the positioner and a feedback response received from the valve actuator. The present invention specifically relates to valve positioners of the type having a pilot valve such as a spool valve and a feedback mechanism including a balance beam hingedly secured at one end to the valve positioner and operable at the other end to shift the position of the spool valve.
Such valve positioners, for example, may be designed for use with a control system including a flow sensor for sending flow rate information to a controller which compares the actual flow rate in the system to the desired flow rate through the system valve. Based on the signal received from the flow sensor, the controller generates a control signal which is converted to a relatively small signal pressure sent to the valve positioner. The valve positioner then causes the flow rate through the valve to change by a specific value which is a function of the signal pressure. The valve positioner further includes a feedback system which constantly operates to return the spool valve to a null position in which the valve positioner acts to maintain a constant flow rate through the system valve until the signal pressure again causes the valve positioner to move the system valve in an open or close direction.
As mentioned above, the present invention is generally directed to valve positioners employing a feedback system operating on force-balance principles. Specifically, the positioner employs a balance beam pivoted at one end and captured between a diaphragm and a compression feedback spring so that its other end operates the spool of a pilot valve, for example. The diaphragm receives the signal pressure to move the balance beam and thus shift the spool valve to supply pressure to the valve actuator thereby changing the flow rate through the system valve. The compression feedback spring operates in response to the movement of the valve actuator shaft to force the balance beam in the opposite direction to balance the signal pressure force. This shifts the spool of the spool valve in the same direction so as to stop the flow of supply air to the actuator and maintain the system valve flow rate.
Often, the balance beam in these valve positioners is secured at one end to the positioner housing by a hinge spring. The stiffness of the hinge spring determines the responsiveness of the balance beam to the signal pressure and therefore affects spool valve shift and flow area for a given change in signal pressure to the diaphragm, In other words, along with the orifice size of the spool valve, the stiffness of the hinge spring determines the sensitivity or "gain" of the valve positioner. When matching a particular valve positioner to a valve actuator, the gain of the positioner is an important consideration. For example, small volume actuators require lower gains to prevent overshoot and oscillation by the actuator and large volume actuators require relatively higher gains to increase the sensitivity and operating speed of the actuator.
Two illustrative examples of valve positioners utilizing balance beams secured to the positioner by a hinge spring are found in U.S. Pat. No. 2,588,988 to Robins and U.S. Pat. No. 2,679,829 to Gorrie et al. The patents to Robins and Gorrie et al. each disclose a valve positioner operating on force-balance principles and including a balance beam hingedly connected at one end to the positioner housing by a leaf or hinge spring. The balance beam is connected to a pilot valve at the other end and is operated through a control pressure sent to a bellows located intermediate the hinge spring and the pilot valve.
Both Robins and Gorrie et al. fail to provide a manner of adjusting the gain of the positioner. Although it is possible and it has been known to change the hinge spring to a thinner or thicker hinge spring in order to vary positioner sensitivity or "gain", this practice is difficult and time consuming. In this regard, to enable a wide range of gain adjustability, a large number of hinge springs of differing thickness would have to be stocked and then fastened in place, one by one, until the desired gain is achieved.
In this regard, certain fluid systems such as those employing relatively large valve actuators require valve positioners having a higher "gain" or sensitivity. That is, for a given signal pressure the balance beam must move the pilot valve of the positioner a relatively large amount. This translates into utilizing a relatively thin or flexible hinge spring which will allow the balance beam to move a greater amount per unit of signal pressure than would a thicker, more rigid hinge spring. Conversely, a relatively thick or rigid hinge spring would be necessary in valve positioners employed in a fluid system requiring low sensitivity or "gain" such as a system employing relatively low volume valve actuators. A thicker, more rigid hinge spring allows the balance beam to move less per unit of signal pressure and therefore causes the pilot valve to be less sensitive to changes in signal pressure.
As previously mentioned, in order to cause a change in the sensitivity or "gain" of past valve positioners it has been necessary to change the hinge spring to a hinge spring having a different stiffness, e.g., to a spring having a different thickness. Any given hinge spring, as used in the past, has produced a single given sensitivity or "gain" in the positioner. Therefore, to allow a range of sensitivities or "gains" a number of hinge springs of differing stiffnesses were required and the variability of sensitivities or "gains" within that range directly corresponded to the number of different hinge springs on hand.
Accordingly, it has been one objective of the present invention to provide a valve positioner having an inexpensive adjustable gain mechanism which allows the valve positioner to be matched to a particular valve actuator without having to stock and interchange parts of the positioner.
Another object of the invention has been to provide infinite adjustment of positioner gain over a wide range of gains while utilizing a single given hinge spring.